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Pulmonary Circulation & Ventilation-Perfusion Ratio

1. PULMONARY CIRCULATION - Overview

• Blood ejected by the heart into pulmonary circulation = blood ejected into systemic circulation (same volume)
• Pulmonary circulation is a low-pressure, low-resistance system
• Total resistance = 1/10th of systemic circulation
• Mean pulmonary arterial pressure = 15 mm Hg (vs. 100 mm Hg systemic)

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2. Functional Organization

• Pulmonary arteries branch parallel to airways
• Pulmonary capillaries are large in diameter with multiple anastomoses
• Each alveolus is surrounded by a capillary basket
• Pulmonary blood vessels = ~40% of lung weight

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3. Functions of Pulmonary Circulation

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4. Special Features of Pulmonary Circulation

1. Low-pressure, low-resistance system
2. Pulmonary arteries have thin walls, less smooth muscle, more compliant
3. Arterioles are thin-walled - ability to constrict is less than systemic
4. Pulmonary veins are thin-walled and highly compliant
5. Capillaries form a lattice in the alveolar wall - not a tubular network; collapse if alveolar pressure exceeds capillary pressure
6. Pulmonary wedge pressure (measured by Swan-Ganz catheter) reflects left atrial pressure
7. PVR falls with increased pulmonary arterial pressure (opposite to systemic) via:
   • Capillary recruitment - collapsed upper-zone capillaries open
   • Capillary distension - compliant capillaries widen

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5. Physiological Significance of Low PVR

• Decreased resistance → slower capillary flow → adequate time for O₂/CO₂ exchange
• Capillary distension → increased surface area → better diffusion
• Prevents excessive capillary pressure → prevents pulmonary edema
• Lowers load on right ventricle

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6. Pulmonary Blood Flow Distribution

• Total blood in pulmonary circulation at any time: ~500 mL
  • Arteries: 150 mL | Veins: 270 mL | Capillaries: 80 mL
• In upright posture: flow increases from apex to base (effect of gravity)

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7. Factors Affecting Pulmonary Blood Flow

A. Pulmonary Vascular Resistance

• Lung volumes - PVR is lowest at FRC; increases at both high and low lung volumes
• Hormones - Serotonin, norepinephrine, histamine, thromboxane A₂, leukotrienes = vasoconstrictors; Adenosine, acetylcholine, prostacyclin (PGI₂), bradykinin = vasodilators
• Oxygen tension - Low O₂ → vasoconstriction (hypoxic pulmonary vasoconstriction)

B. Effects of Gravity - Zones of the Lung (West's Zones)

• At apex: arterial pressure ≈ 6.6 mm Hg
• At heart level: ≈ 14 mm Hg
• At base: ≈ 17.7 mm Hg

C. Regulation of Pulmonary Blood Flow

• Neural: Sympathetic stimulation → mild vasoconstriction; parasympathetic → mild vasodilation (resting sympathetic tone almost absent in pulmonary circulation)
• Hormonal: (see table above)
• Chemical: Hypoxia → vasoconstriction (inhibits K⁺ channels → voltage-gated Ca²⁺ channels open → Ca²⁺ influx)

• Cardiac output, gravity, lung volumes

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8. Filtration Across Pulmonary Capillaries (Starling Forces)

1. Alveolar surface tension - favors filtration (pulls fluid into alveoli)
2. Alveolar pressure - opposes filtration

• Low hydrostatic pressure (8-10 mm Hg) < oncotic pressure (25 mm Hg) → net absorption of fluid from interstitium
• Lymphatics drain excess fluid from peribronchial spaces

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9. Pulmonary Edema

1. Increased capillary hydrostatic pressure (most common - mitral stenosis, left heart failure)
2. Increased alveolar surface tension (decreased surfactant)
3. Decreased oncotic pressure (hypoproteinemia)
4. Increased capillary permeability (ARDS, oxidant damage, inflammatory reactions, neurogenic shock)

• Diuretics, Digitalis, Vasodilators

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10. Drowning

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11. VENTILATION-PERFUSION (V̇A/Q̇) RATIO

• Normal V̇A/Q̇ ratio = 0.8 (ventilation 4 L/min, perfusion 5 L/min)
• Due to gravity: V̇A/Q̇ is high at apex (~3.3) and low at base (~0.6)

Regional Differences

Effects of Changing V̇A/Q̇

• Decreased V̇A/Q̇ (less ventilation relative to perfusion): Alveolar PO₂ falls, PCO₂ rises → venous admixture
• Increased V̇A/Q̇ (less perfusion relative to ventilation): Alveolar PO₂ rises, PCO₂ falls → alveolar dead space

Clinical Importance

• Tuberculosis localizes to apex (high O₂ tension favors Mycobacterium tuberculosis growth)

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12. Venous Admixture and Physiologic Shunt

• Venous admixture: mixing of unoxygenated blood with oxygenated blood
• Caused by low V̇A/Q̇ ratio or a shunt

Types of Shunt

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13. Bronchial Circulation

• Supplies walls of conducting airways (not alveoli)
• Receives only 1% of cardiac output (can rise to 10% in chronic bronchitis)
• Aortic pressure drives it; resistance >> pulmonary
• Physiological importance: Can undergo angiogenesis - provides collaterals to lung parenchyma when pulmonary circulation is compromised

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Key Exam Points (Short Questions likely)

• Special features of pulmonary circulation
• Factors affecting pulmonary blood flow
• Regulation of pulmonary blood flow
• Pulmonary edema causes and treatment
• Ventilation-perfusion ratio and its significance
• Physiological shunt vs. anatomical shunt
• Effects of drowning (fresh vs. salt water)
• West's zones of the lung